MOS transistors typically include source and drain regions in a crystalline semiconductor substrate aligned to a gate electrode that overlies the substrate. A channel region resides in the substrate below the gate electrode and between the source and drain regions. In order to reduce hot electron injection into the channel region, source and drain extension regions are formed adjacent the channel region on either side of the gate electrode. To adequately reduce hot electron injection, the extension regions need to have a very low dopant concentration and a very shallow junction in the crystalline substrate. This is especially important as device dimensions continue to shrink such that the channel width becomes smaller and smaller.
To function effectively, the source and drain extension regions need to have a very low dopant concentration. As the device dimensions shrink, however, the low dopant concentration of the source and drain extension regions tend to increase the electrical resistance, thus undesirably increasing the source and drain series resistance in the MOS transistor. It is therefore desirable to form shallow extension regions that have the necessary low dopant concentration, which not undesirably increasing the electrical resistance.
U.S. Pat. No. 5,966,605 to Ishida discloses a method for infusing dopant into a polysilicon gate structure by first blanket depositing a dopant enriched layer over the wafer after the polysilicon gate structure has been formed. Laser irradiation is then applied to melt the polysilicon and thereby causing the dopant to be infused therein. The laser energy is not sufficient to melt and cause dopant infusion into the source/drain regions.
U.S. Pat. No. 6,372,585 to Yu discloses that nitrogen, implanted into silicon can be induced to bond within the silicon by pulsed laser annealing.
U.S. Pat. No. 6,319,761 to Zhang, et. al. discloses that annealing of ion implanted source/drain regions with an excimer laser improves crystallinity and repairs implant damage.
U.S. Pat. No. 6,365,446 to Chong, et. al., assigned to the present assignee, discloses a method for simultaneously forming silicide contact regions and source/drain regions by first, amorphizing the designated regions by ion implantation of Ge, As, or Ar, next depositing a refractory metal layer, and then implanting the dopant ions through a metal layer. The amorphized regions are then melted by laser irradiation, causing the dopant atoms to quickly distribute in the melted regions. At the same time, the refractory metal reacts with the upper surfaces of the molten amorphized silicon regions to form a metal silicide. The melted source/drain regions then recrystallize to form active source/drain elements.
In related U.S. Pat. No. 6,391,731 to Chong, et. al., a process is disclosed in which both the deep source/drain regions and the shallow source/drain extensions are amorphized using two Ge, As, or Ar implantations. After dopant implantation, a single laser anneal then melts these regions and caused the dopant to distribute. After the anneal, the regions re-crystallize epitaxially from the subjacent single crystalline silicon to form highly activated, very shallow doped regions with abrupt junctions.
U.S. Pat. No. 6,897,118 to Poon et al., assigned to the present assignee, discloses a method in which shallow junctions are formed in source and drain extension regions by first performing and amorphizing implantation using a heavy ion such as silicon or germanium. The amorphizing implantation is followed by introducing a dopant, such as boron, which is activated by pulse laser annealing. The annealing is carried out just below the melting temperature of the substrate. The annealing process results in without altering the dopant concentration profile in the substrate.
While non-melt pulse laser annealing can be carried out to substantially avoid dopant diffusion in the substrate, less than satisfactory dopant activation can result. To maintain optimum electrical resistance, substantial dopant activation needs to be achieved. Accordingly, a need existed for an improved shallow junction fabrication method in which substantial dopant activation is achieved.